I've read the example in the D-Documentation, but there isn't explained how to
import classes from a DLL written in C++
Is it possible to call C++-Classes without a factory function?

No. It's not even possible in C++.
D can only call C++ classes via COM. This is not D's fault, the same
thing is true of C++. For example, you cannot call a C++ class in a
Borland C++ DLL from Microsoft C++. The only time it works at all is the
special case where the executable and the DLL were both compiled with
the same compiler. Utterly ridiculous.